1. Field of the Invention
The invention relates to a system for detecting marks which each comprise at least one marking signal of a given frequency and which together with signals corresponding to information are recorded on a magnetic tape which extends between a supply reel-hub and a take-up reel hub. During recording the tape is driven only via the take-up reel hub which is then driven with a constant rotational speed. The marking signals identify specific items of the recorded information. The system comprises a supply mandril and a take-up mandril for driving the supply reel-hub and the take-up reel hub, at least one magnetic head for reproducing the recorded signals and a frequency-comparison means for comparing the instantaneous frequency of the signals reproduced by the magnetic head with a reference frequency and which when a given relational condition is satisfied supplies a signal indicating the presence of a recording mark.
2. Description of the Related Art
Such a system is known from a commercially available dictation apparatus for magnetic tapes accommodated in cassettes This apparatus enables speech signals dictated by a user to be recorded on a magnetic tape. In addition, the user has the possibility of recording marks on the magnetic tape to identify specific parts of the dictations, for example the end of a dictation or a part of a dictation where a correction of the dictation or an insert in the dictation is required. A mark for identifying the end of a dictation consists of a substantially sinusoidal marking signal having a frequency of 40 Hz, which is recorded on the magnetic tape at the end of the dictation after the speech signals in the same track. A mark which characterizes a part of a dictation to be corrected consists of the substantially sinusoidal marking signal having a frequency of 40 Hz and a further substantially sinusoidal marking signal having a frequency of 1500 Hz, which signals are both recorded on the magnetic tape at the location of the part of the dictation to be corrected by a substitution for the speech signals in the same track. In order to enable these marks and hence the parts of the dictation marked thereby to be detected the known apparatus comprises a system as described in the opening paragraph, the mere presence of a mark being detected by detecting the recorded marking signal having a frequency of 40 Hz contained in all the marks.
During the recording of the dictations and the marks in the known apparatus the magnetic tape is driven only via the take-up reel hub, which is then driven by the take-up mandril at a constant first speed. For detection of the marks the magnetic tape is also driven via the take-up reel hub only, but now the take-up mandril drives this reel-hub at a constant second speed which is twenty times as high as the first speed. As a result of this the speed with which the magnetic tape is driven during detection is twenty times as high as that during recording, enabling the recorded marks to be detected rapidly. During the detection of the marks this twentyfold speed results in a frequency conversion of the recorded marking signals, so that the marking signals reproduced by magnetic heads during detection have a frequency which is twenty times as high. Since both during the recording and during the detection of marks the magnetic tape is driven via the take-up hub, which in each case is driven with a constant speed, this frequency conversion remains the same over the entire length of the magnetic tape. This means that regardless of the location where it has been recorded on the magnetic tape, for example, the recorded marking signal of a frequency of 40 Hz present in all the marks is always reproduced as a frequency-converted marking signal having a frequency of 800 Hz upon detection. In the prior-art system the mere presence of a mark is detected by detecting the occurrence of the frequency-converted reproduced marking signal having a frequency of 800 Hz, for which purpose the known system comprises a frequency comparison circuit in the form of a filter having a fixed mid frequency of 800 Hz, which constitutes the reference frequency relative which the instantaneous frequency of the signals, comprising the speech signals and the marking signals reproduced by means of the magnetic head, are compared. If the frequencies are equal, i.e. if a frequency-converted marking signal having a frequency of 800 Hz is reproduced by means of the magnetic head, the comparison circuit produces an output signal which indicates the presence of a mark and which is employed to control a device for stopping the tape drive. However, if the frequency conversion varies over the entire length of the magnetic tape, which will be the case if during detection of the recorded marks the take-up hub is not driven with a constant speed but with a varying speed or if the supply hub is driven with a constant or variable speed, the marks cannot be detected with the prior-art system comprising a frequency comparison filter circuit having a fixed mid frequency, because in the case of a varying frequency conversion the frequency of the reproduced frequency-converted marking signal changes but the mid frequency of the filter circuit, which forms the reference frequency, remains the same.